1. Field of the Invention
This invention relates to the production of improved lubricating oils. In particular, it relates to the preparation of stable lubricating oils without sacrifice in color which are highly resistant to oxidation and sludge formation when exposed to a highly oxidative environment.
2. Description of Prior Art
Hydrocarbon lubricating oils have been obtained by a variety of processes in which high boiling fractions are contacted with hydrogen in the presence of hydrogenation-dehydrogenation catalysts at elevated temperatures and pressures. In such processes, there is a consumption of hydrogen. Lubricating oil fractions are separated from the resulting products. Such lubricating oil fractions differ from those obtained by fractional distillation of crude oils and the like, since they have such relatively high viscosity index values that solvent extraction treatments are generally not required to enhance their viscosity index values. Such lubricating oil fractions suffer from the shortcoming that they are unstable when exposed to highly oxidative environments. When so exposed, sediment and lacquer formation occurs, thus lessening the commercial value of such lubricants.
Methods in the art directed to lessening such a shortcoming are exemplified by U.S. Pat. Nos. 3,436,334 and 3,530,061. They teach making a lubricating oil product fraction of hydrocracking resistant to deterioration upon exposure to light and air by contacting the lubricating oil fraction with a solid contacting agent having hydrogenation-dehydrogenation properties under hydrogen pressure (U.S. Pat. No. 3,530,061); and making hydrocarbon lubricating oil resistant to such deterioration by contacting high boiling hydrocarbons with a hydrogenation-dehydrogenation catalyst and hydrogen (with hydrogen consumption), and thereafter dehydrogenating the resultant product on contact with a metal oxide or with metal and oxygen (U.S. Pat. No. 3,436,334). Both methods employ hydrogen atmosphere, high pressure and high temperature, i.e. 500.degree. F. to 1000.degree. F. No sulfur is employed in either patent method.
U.S. Pat. No. 2,914,470 is directed to hydrorefining a petroleum oil fraction by contacting it with a catalyst in the presence of hydrogen sulfide. Temperatures and pressures taught for the process of this patent are 600.degree. F. to 825.degree. F. and 150 psig to 3000 psig, respectively.
U.S. Pat. No. 2,432,440 is directed to a high temperature, high sulfur treatment of lubricating oil stocks to improve oxidative stability.
The present invention is directed to a process and means for effecting substantial improvement in oxidative properties of lubricating oil by a low pressure, low temperature contacting with a small amount of added elemental sulfur in the absence of a catalyst followed by removal of unreacted added elemental sulfur. Intrinsic, organically-bound sulfur present in the oil is not substantially removed by the second step of the present process.
U.S. Pat. No. 2,604,438 teaches a "hydroforming" process for catalytic dehydrogenation of light (i.e. boiling at less than 600.degree. F.) hydrocarbon oils, presumably to increase aromatic content. The patent discloses the known fact that in processes of that nature, the presence of a small amount of sulfur in the feed has a beneficial effect. It further states that when the oil to be "hydroformed" has no sulfur, i.e. no sulfur in the light hydrocarbon feed, then a small amount of sulfur, e.g., a reducible sulfur compound, is added to the feed. The patent emphasizes that the invention disclosed therein is only advantageous when the process is carried out at a temperature of at least 825.degree. F.
The prior art practices of hydrofinishing and hydrotreating as a means of treatment of lubricating oil stocks (i.e. stocks boiling at temperatures over 600.degree. F.) leave behind the unstable oil fractions, i.e., hydroaromatic compounds, with labile hydrogen atoms such as, for example, fluorenes, benzofluorenes, acenaphthenes, tetralin, fused cycloalkylaromatics and naphthenes, which are quite unstable toward oxygen, particularly in the presence of metals in lubricating oil formulations containing overbased additives. These hydroaromatic compounds with labile hydrogen atoms are known to be present in small quantities in conventionally furfural refined stocks and can lead to oxidative instability of any lubricant containing them. Further, it is well known that the sensitivity of certain lubricating oils toward alkaline additives can cause oxidative degradation in applications where overbased additives are used, such as automotive and diesel lubricants. Also, metal sensitivity can be quite detrimental to the oxidative stability of lubricants or functional fluids in applications such as turbine circulating oils, steam turbine oils and hydraulic fluids. No method is known at present which so effectively and easily alleviates the above problems as the present two-step invention.